1. Field of the Invention
The subject invention is directed to recovering data as recorded on magnetic media without prior knowledge of the write channel circuitry utilized in the recording thereof. More specifically, the present invention relies on the characterization of the magnetoresistive transducer operating at the interface of the read channel circuitry and the magnetic media. The subject method effectively removes intersymbol interference that, after appropriate decoding, will lead to the recovery of otherwise unreadable data.
2. Description of the Prior Art
In recent years, an increase in commercial activity has been undertaken in the field of hard disk data recovery after a catastrophic failure of the disk drive systems. Currently, the dominant read detection technique in disk drives is the partial response maximum likelihood (PRML). This channel is based on the linearity assumption and prior knowledge of the shape of the ideal readback signal from an isolated transition. By using superposition of properly time-shifted ideal readback signals of isolated transitions, all possible readback signals can be reconstructed and subsequently compared with the equalized readback signal by using maximum likelihood sequence detection criteria. The reconstructed readback signal that best approximates the equalized readback signal is then selected as the most probable binary sequence that was written.
While PRML has been a very successful read channel detection technique that effectively deals with the adverse effects of intersymbol interference (ISI), it is not suited as a read detection method for the recovery of unreadable hard disk data caused by mechanical failures of the drive systems. In order to comprehend why this is the case, it is first necessary to understand the usual conditions of mechanical disk drive failures. Mechanical failures of hard disk drives are often characterized by a head crashing onto the disk, thus damaging both itself and the media. In this scenario, the drive data will be unreadable even when the damaged head is replaced by a new one. This is because the new head will suffer from the same damage once it passes the scratched region on the media, which will surely come to pass when the head actuator moves rapidly in and out across the surface of the disk platter during the initial booting of the hard drive. In the case of a head crash, no recovery software can come to the rescue. Moreover, the use of such software often exacerbates the damage and makes further recovery even more difficult.
Another example of a mechanical failure of the drive is that of a spindle motor which ceases to spin for any of a variety of reasons. It should be clear that no software can be used to recover the data when the spindle motor has failed in that there would be no relative motion between the magnetoresistive head and the magnetically polarized regions on the surface of the disk. Such relative motion is necessary, of course, as it is the change in magnetic flux at the read head, as caused by the relative motion of the magnetically polarized regions on the rotating disk and the read head, that produces a voltage signal in the read channel, which is subsequently processed to produce, ideally, the originally written data.
Under the above failure conditions, the only hope of recovering the recorded data is to open the damaged drive and move the disk platter to another device capable of performing hard disk reading functions. In the new reading environment, however, a read channel other than PRML is required. This is because PRML will only work if the proper time shifts of superimposed ideal transitions are known on the basis of a priori information of the write channel characteristics. This information, however, is often unavailable in data recovery practices when the data must be recovered from numerous disk drives of various, even unknown, origins. Indeed, prior knowledge of the write channel that is necessary for the PRML read channel to function correctly, such as bit cell period, is not available in this foreign read setting. Hence, in order for the hard disk data to be recovered, a new read channel must be devised that is independent of the write channel by which the data were recorded.
Intersymbol interference is one of the main limiting factors in the recording of data on magnetic media as the increase of linear data density and the decrease of bit error rate of hard drives continue to be the thrust of hard disk drive development. As future hard drives entail a much higher track density, it can be expected that ISI will become an issue in the cross-track direction as well as the along-track direction. That is to say, a two-dimensional interference problem that consists of ISI in the along-track direction and ITI (intertrack interference) in the radial direction will occur. However, PRML does not have an easy generalization to the two-dimensional case and the need for a technique to combat two-dimensional ISI presents itself. Such a system would be instrumental for the retrieval of erased or overwritten data that are usually preserved at the edges of erased tracks. While current hard disks have blank regions of zero magnetization (guard bands) between tracks to minimize ITI during the readback process, there are no guard bands in the case of overwritten data. In this case, the new data (overwrite data) are written directly, with a small radial misregistration, on top of the old data (overwritten data), thus creating an across-track ISI between the new and old data as well as an along-track ISI between the adjacent transitions of the old data. A system capable of data recovery at the edges of data tracks would have a strong potential for massive data recovery of purposefully overwritten or erased data from disk drives. These recovered overwritten data, regardless of whether the data were altered inadvertently or intentionally, may have significant implications for the intelligence, security, and law enforcement communities.
Existing PRML data recovery techniques are costly in terms of the hardware implementation thereof. First, PRML has two relatively independent parts: partial response (PR) equalization and maximum likelihood (ML) detection. It is essential to note that the PR part does not eliminate ISI, but intentionally introduces and controls it. The purposefully introduced ISI in the PR process is then used by the ML detector for the selection of the most likely data sequence that has been rewritten. Therefore, strictly speaking, PRML does not remove or mitigate ISI, but deals with it. In other words, PR (for equalization) and ML (for detection) have to be implemented together, which adds to the cost and complexity of the hardware implementation.
PRML is based on the superposition of ideal isolated-transition readback signals and subsequent matching of such synthetically constructed signal to the equalized readback signal. Because PRML is based on the linearity assumption, it does not account for hard and soft transition shifts, neither does it account for non-linear transition shift, which is becoming more severe with increasing data density. In order to deal with such shifts, more advanced forms of PRML channels are needed. Their implementation however requires complicated hardware circuitry that involves PR equalizers, ML detectors, digital filters, and sophisticated clock and gain recovery circuits. Because of the complexity of the circuitry involved, PRML consumes a lot of power, making it not suitable for low-power mobile applications. This would limit the use of PRML in cost-effective and low-power hardware implementations in future hard disk drives.